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The FAA is warning (PDF) that GPS service in a 330 nautical mile circle of Nevada could be "unreliable or unavailable" for six-hour stretches from May 16-27 as broadband wholesaler LightSquared tests whether a signal from one of its proposed 40,000 towers upsets satellite navigation. The test transmitter is 1.6 nm from the Boulder City VOR on the 188.9 degree radial and the warning on the 115-nm radius applies all the away up to FL300. Pilots planning a trip through there are urged to be extra vigilant about NOTAMs as there doesn't appear to be an advance schedule for the tests. "The NOTAMs discussed in this advisory may change with little or no notice," the FAA warns. " Pilots are advised to check NOTAMs frequently for possible changes prior to operations in the area. NOTAMs will be published at least 24 hours in advance of any GPS tests. As we have reported extensively, LightSquared is proposing to build a network of broadband Internet towers across the U.S. that will use a band of radio frequencies right next to those used by GPS satellites and receivers. The FAA notice appears to be using an abundance of caution.

Tests conducted by the GPS industry suggested receivers might be affected at five miles or so, which was enough to raise alarms. LightSquared has said the tests are all about figuring out whether GPS will be affected, and its ability to build its network is conditional on the technical success of those tests. The big worry from the GPS industry is that while LightSquared might be able to tune its transmitters precisely enough to stay out of the GPS frequencies, the sheer power of the broadband signals and their proximity may still affect GPS, which, of course, comes from more than 10,000 miles away in space. If that happens over Nevada, the FAA wants to know about it. "Pilots are strongly encouraged to report anomalies during testing to the appropriate ARTCC to assist in the determination of the extent of GPS degradation during tests," the advisory states.